Re: [PATCH 1/2] NFSD: Use nfsd_iter_read() when ->splice_read is not zero-copy

[Ameer Hamza <ameer.hamza@truenas.com>]

On Tue, Aug 18, 2026 at 07:42:01AM -0700, Christoph Hellwig wrote:
> On Tue, Aug 18, 2026 at 02:08:10AM +0500, Ameer Hamza wrote:
> > For some files splicing a READ cannot avoid a copy: gfs2, kernfs
> > and the cifs direct-I/O modes use copy_splice_read() as their
> > ->splice_read, and the VFS substitutes it for DAX files.
> > copy_splice_read() allocates a fresh page for every page of
> > payload and reads into it; nfsd_splice_actor() then installs
> > those pages in rq_respages, displacing Reply pages the thread
> > already owns. Both sets of pages are then freed.
> > 
> > Route these READs through nfsd_iter_read() instead. It performs
> 
> No.  None of the above are sensible use cases for NFSD exports.
> There is no good reason to add a barely maintained special code path
> thae pokes into internals for this.
>

Yeah, agreed on the poking into internals part. I will rework it
in v2 based on Neil's FMODE suggestion unless you have something
else in mind.